Tethered marine stabilizing system

ABSTRACT

A buoyancy assembly suitable for supporting, either alone or in combination, a load deck or other marine body, said assembly comprising: 
     i) a first portion adapted to be connected to or form an integral part of the marine body, said portion incorporating a piston; 
     ii) a second portion adapted to envelope the piston and thus create a variable volume chamber, the first and second portions being moveable with respect to each other; 
     iii) sealing means adapted to form a fluid tight seal between the piston and the second portion; 
     iv) a constant pressure source adapted to maintain a constant pressure within the variable volume chamber; 
     v) tether means adapted to tether the second portion to the sea bed; 
     wherein the buoyance assembly is adapted such that displacement of the marine body from its hydrostatic equilibrium position results in the generation of a restoring force, upwards or downwards, tending to restore equilibrium.

FIELD OF THE INVENTION

The invention relates to a buoyancy assembly suitable for use withmarine bodies. It is particularly applicable to marine bodies thatpenetrate the water surface, such as oil and gas facilities and bridges,fully submerged bodies and bodies which rest on the sea bed.

BACKGROUND TO THE INVENTION

There have been a number of ways that stability has been provided for inmarine structures in the past.

Floating facilities either ship-shape or column stabilised, achievestability through changing buoyancy by a part of their body moving in orout of water. It is because of this requirement that a large part oftheir body providing buoyancy and stability is exposed to maximumenvironmental loading.

Submarines achieve static stability by ballast control using the powerof pumps to pump water in or out of the vessel as required.

Non-floating structures are stabilised either by piling to the sea bedas foundation or by being attached to large weights known as gravityfoundations.

There are also tethered structures which have tethers stabilised bygravity or piling, acting against the buoyancy of a floating vessel andkeeping the tethers always in tension. The stability of the vessel iseither provided by the tension in the tethers or a combination of thetension as well as the buoyancy changes due to the vessel coming in orout of the water.

Providing piled stabilised foundation is expensive, requiringspecialised crane vessels, pile driving hammers and the expense of thepiles. Gravity stabilised foundation require large, usually concrete,structures and expensive ballast systems. These structures need to beeither externally stabilised for transportation and installationoffshore or their stability element would need to be water surfacepiercing attracting environmental loads. Structures sitting on the seabed fully submerged would need crane vessels to lower them down or raisethem up and still require to be stabilised by a foundation.

Floating facilities which are utilised as offshore platforms for mineralproduction require to keep station whilst being connected to the sourceof the minerals. However, as they need to be surface piercing for theirfunction and stability, they are at times subjected to severeenvironmental loads. In order to minimise their motions and for stationkeeping a number of facilities have been developed. These include:

Dynamic positioning systems which use trusters to resist the waveforces. These are also used to turn a ship around to face the waves.Turrets are required to allow the ships to turn around a moonpoolhousing pipes connected to the source of the minerals.

Tensioners can be used but these have the expense of the foundation aswell as the tensioners.

In summary, the known technology is expensive to install and expensiveto operate on a day-to-day basis. No one technology can provide buoyancyand stability when partially and fully submerged as well as providingfoundation stability to a structure sitting on the sea bed. No oneexisting marine stabilising system has the facility to alter its dynamiccharacteristics to suit changes in the environmental loads.

The closest prior art known to the applicant is the inventor's ownearlier application PCT/GB95/02883. This describes a buoyancy assemblycomprising the first unit and a second unit which, in combinationcontain a volume of compressed gas. Both the first and second units arefree to move up and down in order to vary the volume of compressed gascontained therein.

Whilst this arrangement can maintain hydrostatic equilibrium it tends torequire active rather than passive ballast control to be effective. Thisrequires expensive equipment and complex control circuits.

It is the object of the present invention to overcome some or all ofthese disadvantages.

This invention relates to buoyancy assemblies that can providehydrostatic stability even when fully submerged, and which have thefacility for altering their dynamic characteristics and provide sea bedfoundations that minimise or eliminate the vertical loads acting on thesea bed.

SUMMARY OF THE INVENTION

According to the present invention there is provided a buoyancy assemblysuitable for supporting, either alone or in combination, a load deck orother marine body, said assembly comprising:

i) a first portion adapted to be connected to or form an integral partof the marine body, said portion incorporating a piston;

ii) a second portion adapted to envelope the piston, the first andsecond portions being moveable with respect to each other, therebycreating a variable volume chamber;

iii) sealing means adapted to form a substantially fluid tight sealbetween the piston and the second portion;

iv) a constant pressure source adapted to maintain a constant pressurewithin the variable volume chamber;

v) tether means adapted to tether the second portion to the sea bed;wherein the buoyancy assembly is adapted such that displacement of themarine body from its hydrostatic equilibrium position results in thegeneration of a restoring force, upwards or downwards, tending torestore equilibrium characterised in that that constant pressure sourceis provided by connecting the variable volume chamber to atmosphere.Atmospheric pressure represents a virtually infinite source of constantpressure and is immediately available if suitable connections are made.Preferably the connection to atmosphere is provided by pipeworkconnections through the marine body supporting structure. Inparticularly preferred embodiment the piston is substantially circularin shape and the second portion takes the form of an inverted circularcylinder, closed at one end.

Preferably the diameter of the cylinder decreases towards the open end.The constricted mouth of the inverted cylinder acts as damper due to therestricted water flow. The ratio of the area of the piston face to thearea of the open end of the cylinder is typically 10:1. However, otherratios are possible.

Preferably the second portion is of sealed, double skin construction,natural buoyancy being achieved by enclosing gas within the sealedvolume thus created. The distance between the skins and hence the volumecreated can be considerable, providing the second portion withsubstantial natural bouyancy, keeping the tethers in tension.

Preferably the first portion substantially encircles the second portionunder conditions of minimum buoyancy.

The present invention extends to a marine body incorporating a buoyancyassembly as described herein and to a method of stabilising a marinebody comprising the steps of:

a) providing one or more buoyancy assemblies as described;

b) attaching the or each buoyancy assembly to the marine body;

c) providing control means adapted to control the operation of the oreach buoyancy assembly such that the marine body is stabilised at thedesired point of hydrostatic equilibrium.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be more particularlydescribed by way of example only, and with reference to the accompanyingdiagrammatic drawings in which:

FIG. 1 is an elevation view showing a typical use for the buoyancyassembly, in this case a floating platform where virtually all thehydrostability is provided by the innovative units fully submerged at anequilibrium depth;

FIGS. 2 to 5 show various cross-sectional views of the innovative unitsexposing their working components;

FIG. 6 shows an off shore facility with four stabilising pods in theform of fully submerged pontoons;

FIG. 7 shows an off shore facility with three stabilising pods and acentral column supporting the off shore facility wherein one of theinnovative pods is shown with the external shell structures partlyremoved;

FIGS. 8 and 8A show the pods in greater detail, FIG. 8A illustrating aversion without a central guide means;

FIG. 9 shows a cross-section through a typical pod a maximum buoyancywith the anchor deployed;

FIG. 10 shows a cross-section through a typical pod at minimum buoyancywith the anchor retrieved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following aspects and embodiments of the present invention representthe best ways known to the applicant of putting the invention intopractice. However, these are not the only ways in which this can beachieved and are described here by way of example only.

The general principles underlying a buoyancy assembly of this generaltype have been described in PCT/GB95/02883 the text of which isincorporated here by reference. It is specifically intended that theinventive principles underlying this earlier disclosure should be andare part of this disclosure and this current invention.

The first embodiment of the invention is shown in FIG. 2 showing the twofundamental units in their in-use configuration. Unit 1 consist of aninner chamber marked 301 and a depending closed-ended cylinder 303. Unit2 envelopes the piston of unit 1 creating variable volume chamber marked302 maintained at a constant pressure. These chambers are connected to aconstant pressure source via openings 352.

In the present invention there are important modifications to the priorart. First, the provision of the constant pressure chamber above thepiston. Second, closing the end of the unit 1 central column marked 303.Third, unit 2 is now tethered to the sea bed under tension.

The outer shell of unit 1 may optionally be extended to shroud unit 2and minimise the effect of the environment on it. The shroud typicallyhas inclined slatted openings marked which allow free water flow throughthe inclined openings whilst stopping current or waves to effect thearea inside. The bumpers/rubbing strips, marked 309 are used to allowunit one to rest on the buoyancy tanks, marked 310 of the unit 2 fortransportation. The constant pressure chamber is sealed off from the seavia air tight seals in the plunger chamber marked 307.

Unit 2 consists of the buoyancy tanks marked 310, the optional activeballast tanks marked 319-328, plunger pipe marked 312, plungers marked313 and plunger opening marked 314. The plunger openings are provided asperforations in the plunger pipe 312 close to the plunger 313. Theseperforations allow the free flow of gas to maintain chamber 302 atconstant pressure. The plunger is free to move vertically within theplunger chamber with the constant pressure air above the plunger sealedfrom the sea below. The active ballast tank is open to the sea at thebottom through the inclined slatted openings marked, 316. The opening isslatted to minimise the effect of the environment in the tank. The levelof water in these tanks is dependant on the pressure from the sea at thelevel of the openings, and the compressed air pressure acting on theballast water surface marked 317. Therefore, any change in the airpressure in the plunger chamber due to the movement of the plungerswould affect the air pressure within the tank and thus the level of theballast water.

Unit 2 could also have a skirt which is open ended and providesadditional hydrodynamic resistance and also if set on the sea bed itwould penetrate into the soil and provide sliding stability. In thiscase the bottom of the tanks would act as mud mats on the soil. The unit2 is attached by anchors or other tether systems marked 345 to anchorsor some other heavy object 336, which could have its own ballast system.The anchor chains or tethers are extended using winches or other tensiondevices. In operating conditions when the unit is at hydrostaticequilibrium, these tethers provide a constant tension against the excessbuoyancy provided by the unit 2 keeping the anchors or tethers intension at all times.

Unit 3 is the air vent and compressed air supply system for so-calledactive ballasting. This system has been described in detail inPCT/GB95/02883 and is incorporated herein by reference.

When unit 2 as described above is tethered to the sea bed then unit 2assumes a positive buoyancy, any downward movement by unit 1 willinitially be resisted by this positive buoyancy.

Any upward movement of unit 2 will be resisted by the tether tensionwhich is dependent on the stiffness of the tether itself.

It will thus be appreciated that the positive buoyancy in unit 2 is theexcess buoyancy developed in equilibrium with the tether. The effectivegives a stiffer, more stable platform. The stability of the platform isno longer dependent solely on the constructional details of unit 2, orunit 1 and unit 2 in combination. Nor is it entirely dependent upon thehydrodynamic damping of unit 2. The result of this modification whichtethers unit 2 under tension to the sea bed means that either unit 3 isno longer necessary or it is only required as a back-up facility. Thiscan greatly simplify the construction of the system with attendantsavings in cost. The tethers effectively turn an active system aspreviously described into a passive system with improved performancecharacteristics.

The basic features of the first embodiment can be summarised as follows.The stabilising assembly is made in two fundamental units. Unit 1 isshown separately as FIG. 3 which essentially consists of an enclosedbuoyancy chamber which is shaped to allow unit 2, described as a passiveballast chamber, to freely slide up and down the central column of unit1.

Unit 1 consists of an enclosed buoyancy chamber marked 301 which isattached to the marine body typically shown in FIG. 1. It would normallyhave a central column designed to allow unit 2 to slide freely up anddown. The plunger column marked 312 and the plungers marked 313 are alsopart of unit 1 and rigidly fixed to it. The plunger column has anopening marked 314 which allows the flow from pipes marked 352 in to theair chamber in unit 2 marked 302. The pipes marked 352 are used tomaintain constant pressure in the air chamber marked 302 by eitherhaving an opening to the atmosphere through the marine body marked 353or by a constant pressure supply system marked 351. The active constantpressure supply system can be accommodated totally within unit 1 or canbe accommodated in the deck of the marine body. Controlling the rate offlow in and out of the constant pressure chamber alters the dynamiccharacteristics of the system.

If the required design constant pressure is atmospheric, then theconstant air pressure chamber 351 can be open to atmosphere. Thisprovides, in effect, an infinite source of constant air pressure. Theair chamber 302 is sealed by the plunger marked 313 and seal 307 at thelower portion and a seal marked 350 at the higher portion. Theattachments marked 309 are bumpers when two units come together.

Plunger column 312 is perforated as marked 314 close to the plunger 313.This arrangement provides for the air chambers 351 and 302 to be at thesame, constant pressure, whatever the relative position of plunger 313in the air chamber 302.

Unit 2 is essentially a buoyant system marked 310 which is free to slideup or down with respect to unit 1. The sliding attachments are providedby the sliding joints marked 315, which in this case are shown aroundthe central column of unit 1. Unit 2 is essentially a self stabilisingpassive ballast chamber. The unit 2 is stabilised in one or acombination of the following two ways:

1. Having anchor chains or tethers marked 335 attached to anchors orother fixed foundations marked 336 on the sea bed. It would have apositive buoyancy in its equilibrium position against a tension in theanchor chains or the letters.

2. Having low and high pressure supply system marked as 319, 320, 321,322, 323, 324, 325 and 326 actively adjusting its buoyancy.

Unit 2 has a ballast chamber which has an opening to the sea marked 316.The hydrostatic water pressure in the ballast chamber acts on the bottomsurface of the plunger providing additional buoyancy force for unit 1.This chamber can either be full of water up to the plunger or can havecompressed air in chamber marked 311. Providing air in this chamber hasthe disadvantage that the air is compressible and this may cause someloss of buoyancy, but it has the advantage of keeping the plunger 313and the seal 307 within an air medium and also acts as an air cushion,damping the effect of the loading between the units. It will bedesignated based on reliability and cost considerations. This simplifiesthe engineering design of the system considerably.

The fundamental principals of the system are described as follows:

In equilibrium position unit 1 has adequate buoyancy to support its ownweight and that of the marine body deck and cargo. This buoyancy forceis partly due to displaced water volume of unit 1, other submergedenclosed structures attached to it and the force acting on the plunger.The force acting on the plunger is due to unbalance of pressure on thelower and upper surfaces of the plunger. The pressure on the lowersurface is at hydrostatic head of the sea and the pressure on the top iskept constant, typically at atmospheric. When the marine body isdisturbed as shown in FIG. 1 unit 1 would either be pushed up or down.When pushed down, as in FIG. 4, the plunger would move downwards withinthe ballast chamber. This is because unit 2 is held under tension asexplained above and would also resist movement due to its own inertiaand hydrodynamic drag. The unit 2 would also be subjected to a lowerlevel of loading being furthermost away from wave zone. When the plungermoves down the hydrostatic pressure on the lower surface will go upwhilst on the upper surface the pressure will remain constantessentially by sucking air from the atmosphere. This will increase thebuoyancy force acting on unit 1 until the out of balanced force isstabilised. Movement of the plunger within unit 2 does not alter thebuoyancy of unit 2 as long as the pressure in the air chamber marked 302is maintained.

If the plunger moves upwards with respect to unit 2 then the hydrostaticpressure acting on the lower surface of the plunger will be reducedwhilst the pressure on the upper surface will remain constant byessentially expelling air in to the atmosphere. This will reduce thebuoyancy of unit 1 until equilibrium is reached.

A further embodiment of the application of the present invention isshown in FIGS. 6 to 10. FIG. 6 shows an offshore facility with 4stabilising pods, fully submerged to pontoons and small diameterstructural columns supporting a drilling facility, accommodation andcargo. FIG. 7 shows an offshore facility with 3 stabilising pods and acentral column supporting an offshore facility. In FIG. 7 one of theinnovative pods is shown with the external shell structures partlyremoved allowing a clear view of the inside of unit 2 showing the pistonwhich is fixed on to unit 1 and the marine body. This pod is shown moreclearly in FIG. 8. FIG. 8A shows a version of this type of pod in whichthe central guide means passing down the centre of the plunger or pistoncolumn has been removed. This leaves the full internal diameter of theplunger column as a passageway for the movement of constant pressure airor for access.

The advantage of this overall arrangement is that the seal around thepiston column and unit 2 is no longer required. This is because in thisarrangement the unit 2 passive ballast chamber opening marked 316 isutilised for the piston column. The seal 307 around the plunger is shownas a flexible rubber cylinder, one end attached to the circumference ofthe piston whilst the other end is attached on to the inside wall of theunit 2. The differential pressure below and above piston forces therubber cylinder to loop, one side being forced flat against the insidewall of unit 2 the other side stretching in towards the inside of theconstant air chamber above the piston. As the piston move up or down,the rubber seal ravels or unravels itself on to the wall of unit 2. Therubber cylinder can be made of a number of layers for added safety. FIG.9 shows this arrangement with maximum buoyancy with the piston fullyextended with respect to unit 2 and the anchor being lowered. FIG. 10shows this arrangement with minimum buoyancy with the piston at isuppermost position with respect to unit 2 and the anchors retrieved.

In this second embodiment the structural features have in effect beeninverted with respect to the first embodiment. The single, largediameter piston 313 coupled with the relatively small diameter port 316through which displaced water must pass provide much improvedperformance characteristics.

The flexible seal 307 can be constructed from any suitable plastics,rubber or other material as selected by the material specialist.Alternative sealing arrangements are also possible and this disclosureis intended to encompass all forms of gas/water-tight seals or sealingsystems.

It will be appreciated that the force that restores hydrostaticstability for floating vessels is dependent on the water displaced bythe body as it moves in and out through the surface. Consequently, alarge part of the body of conventional floaters is required to be nearthe surface where the environmental conditions are the harshest. Thesize of this water plane area is governed by the stability requirements.

This new innovation provides a hydrostatic restoring force passivelygenerated by a submerged body. This submerged stability systemessentially allows the design of floating vessels which are not limitedto floating on the surface, but can also float at a distance below.

The system negates or minimises the exposure to the elements making itpossible to design facilities such as reusable virtually fixed floatingvessels, storage facilities below the wave zone and self installingreusable sub-sea installations.

The marine stabilising system comprises in essence two portions adaptedto move vertically with respect to each other. A constant processchamber and a passive ballast system is used to generate a restoringforce. For water depths up to 50 m below the main sea level the constantpressure chamber can be at atmospheric pressure, being directlyconnected to the atmosphere. When one portion moves with respect to theother the bellowing action either takes air in or expels it. The changein the displaced volume provides the static restoring force.

Computer simulations have been carried out to investigate the possibleperformance criteria of this new inventive concept. The analytical toolswere based on PHOENICS computational Fluid Dynamics programme. Theseallowed simulation of the dynamic behaviour of each component, the fluidflows and investigation of the phasing relationship between them. Anumber of parametric studies were run covering different waterdisplacements mass distributions and anchor spring stiffness. Theloading conditions included regular and irregular waves in combinationwith variable static (cargo) and/or dynamic loads such as the crane orthe derrick loading.

The results proved carefully that the fully passive submerged systemwould provide a stabilising force comparable to the conventional surfacepiercing elements for all the loading conditions.

FIG. 6 shows a conceptual design of a typical offshore facilityincorporating the new innovation and demonstrated that the requiredrange of stability can be achieved. The design has the same deck weight,pontoon weight, operating displacement, and variable deck loading as apopular semi-submersible drilling vessel.

The column diameter of the conventional semi-submersible is 12 m basedon the stability requirements. The top of pontoon elevation is at 10 mbelow the main sea level. The top of pontoon elevation of the FIG. 6design is at 30 m below main sea level and the columns are 3 m diameterbased on the structural considerations. The stability being provided bythe passive system which are incorporated in to the pontoon.

This study also demonstrated the design would satisfy the requiredstability criteria when in transit, in operating draft and duringtransition to and from the operating draft. At these diameters, the waveand current loading are inertia-dominated, which is proportional to thesquare of the diameter. The waves also loose 95% of their energy withinhalf their wave length from the surface. For most operating waves thisis approximately 50 m within which this new design showed up to 90%reduction in wane loads when compared to the conventionalsemi-submersible.

There have been major developments in the floating, and reusablefacilities, such as the turet based monohulls, tension legs, spar andjack up based platforms. However, none have solved the problem inherentin all conventional floaters—their exposure to large environmentalloads.

In the context of this disclosure the terms piston and plunger have abroad meaning. They are intended to encompass any shape or constructionwhich can act to vary the volume of fluid beyond the plunger head.Whilst a circular-cylindrical arrangement as illustrated, any suitablesize or shape of plunger will suffice.

For the avoidance of doubt, the invention is intended to encompass anytethered buoyancy assembly which contains a submerged buoyancy chambermaintained at a substantially constant pressure, the assembly beingadapted such that movement of or about a piston or plunger causes achange in the submerged volume of said constant pressure chamber,thereby developing a stabilising hydrostatic force.

It will also be appreciated that the chamber above the piston, forexample chamber 302, is the one which must be maintained at constantpressure for maximum efficiency. Importantly, this chamber can bemaintained at a different pressure to the internal pressure within unit1 or within the closed section of unit 2.

The preferred constant pressure will be determined by the operating anddesign perameters of each particular installation.

What is claimed is:
 1. A buoyancy assembly suitable for supporting,either alone or in combination with one or more further buoyancyassemblies, a load deck or other marine body, said assembly comprising:i) a first portion adapted to be connected to the marine body, saidportion incorporating a piston; ii) a second portion adapted to envelopethe piston and thus create a variable volume chamber, said first andsecond portions being movable with respect to each other; iii) sealingmeans adapted to form a fluid tight seal between said piston and saidsecond portion; iv) a constant pressure source adapted to maintain aconstant pressure within said variable volume chamber; and v) tethermeans adapted to tether said second portion to the sea bed; wherein saidbuoyancy assembly is adapted such that displacement of the marine bodyfrom its hydrostatic equilibrium position results in the generation of arestoring force, upwards or downwards, tending to restore equilibrium.2. A buoyancy assembly as claimed in claim 1 wherein the constantpressure source is provided by connecting the variable volume chamber toatmosphere.
 3. A buoyancy assembly as claimed in claim 2 wherein theconnection to atmosphere is provided by pipe work connections which areinsertable through the marine body supporting structure.
 4. A buoyancyassembly as claimed in claim 1 wherein said constant pressure source isprovided by an active balance system incorporating a compressed gassource.
 5. A buoyancy assembly according to claim 1 wherein said pistonis substantially circular in shape and said second portion takes theform of an inverted circular cylinder, closed at one end.
 6. A buoyancyassembly according to claim 5 wherein the diameter of said cylinderdecreases towards the open end.
 7. A buoyancy assembly according toclaim 1 wherein said second portion is of sealed, double skinconstruction, natural buoyancy being achieved by enclosing gas withinthe sealed volume thus created.
 8. A buoyancy assembly according toclaim 1 wherein said first portion substantially encircles said secondportion under conditions of minimum buoyancy.
 9. A marine bodycomprising: a marine structure; and a buoyancy assembly for stabilizingsaid marine structure comprising: i) a first portion adapted to beconnected to the marine body, said portion incorporating a piston; ii) asecond portion adapted to envelope the piston and thus create a variablevolume chamber, said first and second portions being movable withrespect to each other; iii) sealing means adapted to form a fluid tightseal between said piston and said second portion; iv) a constantpressure source adapted to maintain a constant pressure within saidvariable volume chamber; and v) tether means adapted to tether saidsecond portion to the sea bed; wherein said buoyancy assembly is adaptedsuch that displacement of the marine body from its hydrostaticequilibrium position results in the generation of a restoring force,upwards or downwards, tending to restore equilibrium. b) attaching saidbuoyancy assembly to the marine body; c) . . . equilibrium.
 10. A methodof stabilizing a marine body comprising the steps of: a) providing atleast one buoyancy assembly, said assembly comprising: i) a firstportion adapted to be connected to marine body, said portionincorporating a piston; ii) a second portion adapted to envelope thepiston and thus create a variable volume chamber, said first and secondportions being moveable with respect to each other; iii) sealing meansadapted to form a fluid tight seal between said piston and said secondportion; iv) a constant pressure source adapted to maintain a constantpressure within said variable volume chamber; and v) tether meansadapted to tether said second portion to the sea bed; wherein saidbuoyancy assembly is adapted such that displacement of the marine bodyfrom its hydrostatic equilibrium position results in the generation of arestoring force, upwards or downwards, tending to restore equilibrium.b) attaching said buoyancy assembly to the marine body; c) providingcontrol means adapted to control the operation of said buoyancy assemblysuch that the marine body is stabilized at the desired point ofhydrostatic equilibrium.